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Francis 186-212 Intro. Petrol. 2014
Petrology Lab 8: Metamorphic Minerals & Textures
Stations A1 and A2: Metamorphic Minerals:
These stations contain examples of common metamorphic minerals. Identify each of them and
review their characteristic physical properties.
A1- Low Grade: zeolites, epidote/zoisite, chlorite, chloritoid, actinolite, andalusite,
oikoblastic cordierite
A2- High Grade: hornblende, plagioclase, garnet, staurolite, sillimanite, kyanite,
pyroxenes, blue cordierite
muscovite/paragonite (K,Na)Al2(AlSi3O10)(OH)2
WY2T4O10(OH)2
andalusite
kyanite
sillimanite
Al2SiO5 or Al2OSiO4
Al2SiO5 or Al2OSiO4
Al2SiO5 or Al2OSiO4
Y2OTO4
Y2OTO4
Y2OTO4
staurolite
(Fe,Mg)2Al9O6(SiO4)4(O,OH)2
Y11 O6(TO4)4(O,OH)2
chloritoid
cordierite
(Fe,Mg)2Al4O2(SiO4)2(OH)4
(Fe,Mg)2Al3(Al,Si5)O18.nH2O
Y6O2(TO4)2(OH)4
Y5(TO3)6.nH2O
garnet
(Fe,Mg)3Al2(SiO4)3
X3Y2(TO4)3
chlorite
(Mg,Fe)3(Al,Si3)O10(OH)2(Mg,Fe)3(OH)6 Y6(T4O10)(OH)6
biotite
K,(Mg,Fe)3(AlSi3O10)(OH)2
WY3T4O10(OH)2
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zoisite - epidote
actinolite
Ca2(Fe,Al)3O(SiO4)(Si2O7)(OH)
Ca2(MgFe)5Si8O22(OH)2
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X2Y3O(TO4)(T2O7)(OH)
X2Y5T8O22(OH)2
Francis 186-212 Intro. Petrol. 2014
Stations B1 and B2: Metamorphic Textures:
The names of metamorphic rocks usually contain a term reflecting their metamorphic texture:
slate:
extremely fine-grained rock exhibiting a perfect planar cleavage defined by the
alignment of sub-microscopic phyllosilicates grains. Distinguished from shale by its
greater hardness and the fact that cleavage is generally at an angle to bedding.
hornfels:
very fine-grained dark rock lacking any foliation and commonly exhibiting concoidal
fracture, distinguished from mudstones by hardness, sometimes called argillite.
phyllite:
very fine-grained rock exhibiting a well-defined foliation defined by the alignment of
microscopic phyllosilicate grains. Characterized by a pronounce sheen.
schist:
fine to coarse grained rock exhibiting a wavy foliation defined by the alignment of
visible phyllosilicate grains.
gneiss:
rock in which solid state differentiation has produced alternating felsic (leucosomes)
and mafic (melanosomes) layers.
granulite:
high-grade metamorphic rock characterized by equigranular textures and an absence
of hydrous micaceous minerals. Commonly gneissic. Sometimes superficially looks like
an igneous rock, but the granular as opposed to lath-like habit of the plagioclase is a
giveaway.
migmatite:
high-grade metamorphic rock which has undergone partial melting. Characterized by
mobilized granitic leucosomes with more refractory residual melanosome layers.
marble:
crystalline metamorphosed limestone.
quartzite:
recrystallized quartz arenite, quartz grains are intergrown, rather than rounded. Very
hard.
skarn:
calcium-rich contact-metasomatic rock – composed of calc-silicate minerals
± carbonate formed at the contacts between magmatic intrusions and carbonate
sedimentary rocks.
amphibolite: metamorphic rock consisting largely of amphibole, with lessor plagioclase, commonly
gnessic with a mineral lineation defined by amphibole crystals. Typically basaltic in bulk
composition.
eclogite:
high-pressure rock of basaltic composition dominated by pyropic garnet (Mg3Al2(SiO4)3)
and jadeitic (NaAlSi2O6) clinopyroxene  kyanite, indicator of diamonds. More mafic
compositions with similar mineralogy are termed garnet clinopyroxenite.
mylonite:
fine-grained rock exhibiting fine parallel mineralogical banding containing a
pronounced lineation, sometimes with rotated metacrysts of garnet or K-Spar - produced
by extreme strain. More generally might be termed tectonite.
Metamorphic rocks are usually named according to the observed minerals that best constrain the P-T of of
metamorphic recrystallization, followed by their textural type. For example:
garnet staurolite biotite schist
We will see next week that the assemblage garnet, staurolite, biotite places the metamorphic temperature in the
approximate range of 550 to 650oC.
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